<form action="<?php echo $this->href('/locale/currency/change') ?>" 
      method="post"
      id="currency-form">
<div>
<?php echo $this->formSelect(
    'currency-code', 
    $this->box->getData('currencyCode'), 
    array(
        'onchange' => "document.getElementById('currency-form').submit();"
    ), 
    $this->box->getData('currency')
);?>
</div>
</form>